The Ordinary World, to a person like me who has read 3e only, seems to be a trick of perception. A "lie" that masks the Mythic World's animism with amoral causal systems. But they represent the same truth - the volcano is erupting in both, even though the meaning and cause of that sentence varies wildly between them.

So. Can a human in the Border Mythic perceive and interact with humans in the Ordinary World? Conversely, can a human in the Ordinary World perceive and interact with humans in the Border Mythic?

Yes. Most of the time. Communication is as difficult as for any two people who are in different perceptual worlds—-think of it like a kid trying to communicate with a dog, each having access to senses and ideas that the other doesn't. "Let's go for a walk!" is pretty easy to convey, but "I'm grounded, sorry," or "I smell that you are going to be sick in a few hours" is pretty hard.

How about if you go deeper? Can the Deep Mythic and the Border Mythic interact? Can the Deep Mythic and the Ordinary World interact? I assume that it's not free all the way down, or else the Imperial blockade in the Spirit World would be kind of pointless. But in all three upper layers, aside from the nature of the world's particulars, aren't those particulars essentially equivalent?

I didn't make this terribly clear, for which I apologize.

By the time you get to the Deep Mythic you're not really interacting with person-sized organics any longer unless they've come down with you. So how do you game this out when it comes to somebody trying to interact with you?

I figure it has to be the Auctoritas that separates the Deep and Border Mythic that makes this work. You need to either use a miracle + 3 Strike or a fair bit of concentration to get to the Deep Mythic, even for a Power. (And a miracle + 3 Strike or a lifetime of work to reach the Spirit World.) I'd guess that also applies to limited interactions across the border.

For instance, somebody is sitting next to you. They spent a few hours in meditation and sank into the Deep Mythic. If you did the same, or if you used the right 3 Strike + miracle, you could probably recognize that they're there. Oh, hey, that's not just a part of my environment—-that's Dave! Similarly, with a lifetime of research, you could probably figure out what "actually" happened to Cindy, who fell all the way into the Spirit World. They'd also perceive this from their side as you kind of "manifesting" yourself from the various scattered strands of your impact on the larger world into a coherent intent or attention in their world. Like, you're focusing yourself down from all the systems you participate in into a body long enough to see them or do whatever.

This is a soft ruling; the HG could handle it a little differently if they wanted.